In manufacturing of semiconductor devices, various kinds of processing are done on semiconductor wafers consisting of silicon or the like, and the processing includes oxidization processing to form an oxide film on the surface of the wafer. The formed oxide film functions as a gate-insulating film of transistor chip, for example. The gate-insulating films are needed in recent years to be thin in film thickness together with long-running high reliability.
Generally, the oxide film formation on a wafer is processed by placing a wafer inside a reaction vessel heated to a high temperature, e.g. 750 to 1100° C., in the presence of oxygen under the conditions of normal or reduced pressure. And the oxide film formation method under depressurized conditions particularly has an advantage that the generating rate of oxide film is small due to the low oxygen concentration whereby the more degrees of freedom for controlling the formation of oxide film is easy even if thin film thickness is formed.
However, in such an oxide film formation process in a low oxygen atmosphere at a high temperature, the problem is that the oxide films obtained in the oxide film formation process turn out to have low reliability due to roughness formed on the surface of the wafer by being etched or nitriding by the nitrogen gas for generating the atmosphere of low oxygen concentration during the time period from loading of the wafer into the reaction vessel to the initiation of actual oxide film formation, i.e. during the heating process to raise the temperature of the reaction vessel or the surface of the wafer to a predetermined temperature for oxidation treatment.
In particular, it is difficult to form high-quality oxide films with high uniformity of the film thickness and film quality over the entire area of larger-diameter wafers which are common now.
Given such situations, the following methods are employed to form high-quality oxide films while averting the problems caused by the oxide film formation process in a low oxygen atmosphere at a high temperature.
(1) A method of forming an initial oxide film on the surface of a wafer as a protective oxide film by maintaining the oxidation atmosphere even in the heating process when carrying out the oxide film formation process at normal pressure.
In this method, improved uniformity of an oxide film can be obtained in the oxide film formation process because the protective oxide film is formed on a wafer under somewhat controlled condition in the heating process.
(2) A method of forming a protective oxide film in advance on the surface of a wafer by chemical treatments such as a wet cleaning with hydrogen peroxide solution etc. for preventing oxide films to have low reliability because of the low oxygen atmosphere at a high temperature when carrying out the oxide film formation process at normal pressure.
In this method, degradation of reliability of the oxide film formed in the oxide film formation process becomes controllable because the protective oxide film is formed under controlled conditions. This method is advantageous when raising temperature in a low oxygen atmosphere in order to prevent the film thickness of a protective oxide film from being thick.
(3) A method of forming a protective oxide film with controlled film thickness by controlling the partial pressure of oxygen inside a reaction vessel in the heating process when carrying out the oxide film formation process at reduced pressure.
(4) A method of forming a protective oxide film with controlled film thickness by controlling the partial pressure of oxygen in the heating process by dilution with an inert gas as such as a nitrogen gas when carrying out the oxide film formation process at normal pressure.
However, the above methods have the following problems:
(1) In the method of positively forming a protective oxide film by performing the heating process in the oxidation atmosphere, anticipating the film thickness of the protective oxide film formed in the heating process, which is necessary to accomplish the process of the oxide film formation, is quite difficult since the film thickness of the protective oxide film is difficult to be controlled. For instance, in a case that an ultrathin oxide film with the final film thickness of 2 nm or less needs to be formed in particular, the operating condition would have much less variation in the oxide film formation process.
Although the protective oxide film formed in this way is generally considered to have a poor film quality, the protective oxide film may be reformed in the process of oxide film formation in many cases. However, in a case that an oxide film with the final film thickness of 2 nm or less is formed in particular, forming an oxide film with high film quality would turn out to be difficult due to the difficulty of reforming the protective oxide film sufficiently in the oxide film formation process because the protective oxide film occupies a large portion of the oxide film finally obtained.
Furthermore, in general, a protective oxide film is formed inside a reaction vessel or on the surface of a wafer where the temperature distribution is uneven, therefore, the oxide film finally formed would turn out to provide quite poor uniformity in film thickness since the film thickness distribution of the protective oxide film formed on the surface of a wafer would show a significant nonuniformity corresponding to a nonuniformity of the temperature of the surface of the wafer.
(2) In the method of forming a protective oxide film using chemical means by a wet cleaning, a slight amount of metallic elements would inevitably be introduced into the chemically formed protective oxide film in the process of the wet cleaning, which fact is the cause of low reliability of the oxide film finally formed.
This method also have a problem that organic contaminants are likely to adhere to the surface of a wafer since the wafer has an occasion to be exposed to atmospheric air before the wafer is brought into a reaction vessel after the protective oxide film is formed by a wet cleaning. Although, in some cases, the organic matters adhered to the surface of the wafer can be removed during combustion by executing the heating process in an oxidation atmosphere, uniformity and control of the film thickness of the oxide film finally formed would significantly be degraded and thus the method is not preferable.
Moreover, the protective oxide film formed using chemical means by a wet cleaning has low reliability in uniformity in film thickness within the entire surface of a wafer, in controllability of film thickness and in film quality, and the oxide film finally formed would have low reliability as a result.
(3) The method of forming a protective oxide film in the heating process at reduced pressure is implemented while the partial pressure of oxygen is being lowered, and the protective oxide film itself can be thinner compared to methods implemented at normal pressure. However, although greater degree of freedom can be obtained to control the final film thickness, the problem is that the entire film thickness would be nonuniform since nonuniformity of the wafer surface temperature affects on thickness distribution of the protective oxide film which is formed in the heating period.(4) In the method of forming a protective oxide film with controlled film thickness by controlling the partial pressure of oxygen in the heating process with a nitrogen gas, forming a high-quality oxide film is difficult as stated previously due to a wafer inevitably nitrided.
As stated above, the problem is that a high-quality oxide film cannot be formed with advantage on a wafer by the conventional oxide film formation methods.
The present invention suits to solve the problems stated above, and the purpose of the present invention is to provide an oxide film formation method to form with advantage a high-quality oxide film having excellent uniformity in film thickness and film quality over the entire wafer.
The present invention has been completed based on the knowledge obtained by locating the root cause of the low reliability of an oxide film formed in the oxide film formation process, and the root cause is that a stable protective oxide film cannot be formed under conditions with sufficient control before starting the process of oxide film formation by the conventional oxide film formation methods.